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Editors' Choice

Conservation Pay-Off

Andrew M. Sugden

CREDIT: IAMKASPAR/FLICKR

The steady decline of biodiversity and the increasing homogenization of biotas through human influence has become a familiar theme of the 21st century. Carvalheiro et al., however, suggest that these processes may be slowing down, at least for some groups of organisms in Europe. Focusing on assemblages of flower-visiting insects and plant species recorded in four 20-year time periods from the 1930s onward in Britain, Belgium, and the Netherlands, they show that the rates of biodiversity decline and biotic homogenization were at their greatest in the mid-20th century, a period marked by the most rapid expansion and intensification of agriculture. Since 1990, the rate of change has decelerated, indicating that conservation measures, along with the declining rate of land conversion, are beginning to have a positive pay-off for at least some of the elements of European biodiversity.

Ecol. Lett.16, 870 (2013).

Chemistry

New Life for Cyclopropanes

Jake Yeston

Although the carbon bonds in triangular cyclopropane rings are strained, these substructures appear in numerous, reasonably stable natural and synthetic compounds. Biochemically, they tend to result from enzymatic coupling of olefins with stabilized cations. Chemists instead typically treat the olefins with transiently generated, metal-bound neutral carbenes, a strategy that broadens the versatility of substitution patterns around the ring. Mechanistically, metal activation of carbenes is loosely analogous to the pathway whereby cytochrome P450 enzymes activate oxygen, and this insight led recently to the preparation of engineered P450 variants active for synthetic cyclopropanation in aqueous solution (see Coelho et al., Reports, 18 January 2013, p. 307). Coelho et al. have now further engineered this class of enzymes to enable carbene-derived cyclopropanation in vivo in Escherichia coli cells, despite the complete absence of a native reaction in this vein. The key mutation was replacement of a cysteine residue with serine, leading to O (rather than S) coordination of the iron active site (confirmed crystallographically). This substitution facilitated catalyst activation using endogenous NADH as a reductant. Overall activity for styrene cyclopropanation, with ethyldiazoacetate as the carbene precursor, was even higher in vivo than in vitro, with no loss of enantio- or diastereoselectivity (favoring the cis product).

Nat. Chem. Biol.9, 10.1038/NCHEMBIO.1278 (2013).

Ocean Science

Corals Under Threat

H. Jesse Smith

CREDIT: ELIZABETH D. CROOK

Rising concentrations of atmospheric CO2 are causing the pH of surface seawater to decrease, posing a threat to coral reefs. Crook et al. examined how calcification of Porites astreoides, an important reef-building coral found in the Caribbean, is affected by the acidity of the seawater in which it grows by measuring samples along a transect spanning a natural gradient of pH and aragonite saturation. Coral calcification rates decreased significantly as pH and aragonite saturation decreased, in a manner consistent with that exhibited by the same species in laboratory carbonate manipulation experiments. This indicates that the corals were not able to respond quickly enough to prevent the impacts of local ocean acidification on their skeletal growth and development, a discouraging message considering the hopes that some have put on the ability of corals to adapt to future ocean conditions.

Proc. Natl. Acad. Sci. U.S.A.110, 10.1073/pnas.1301589110 (2013).

Structural Biology

Form and Function

Valda Vinson

Formins are involved in regulating actin polymerization by nucleating new actin filaments and promoting elongation at the filament barbed end. In formins, a donut-shaped dimer of FH2 domains encircles the barbed end of the filament, and the FH1 domain binds profilin-actin complexes and rapidly transfers actin monomers to the barbed end. The FH2 dimer gates polymerization by transitioning between an open and a closed state, with the open state favoring actin monomer binding. Many formin-mediated actin structures experience tension, but how this affects formin function is unclear. Courtemanche et al. explored the effect of tension on actin polymerization induced by yeast formin Bni1p. Formin was anchored to a lipid bilayer through its N terminus, and buffer flow was used to align initiated filaments into “actin curtains.” It has previously been proposed that tension might favor polymerization, by increasing the FH2 domain stepping rate, but limit the enhancement in rate provided by profilin by slowing the transfer of profilin-actin to the barbed end. Courtemanche et al. found the opposite—small forces slowed formin-mediated polymerization in the absence of profilin but increased the rate of polymerization in its presence. Simulations were consistent with the proposal that tension favors the closed state of the FH2 dimer, but profilin-actin bound to the FH1 domain allosterically shifts the FH2 equilibrium toward the open state.

Proc. Natl. Acad. Sci. U.S.A.110, 9752 (2013).

Physics

Sensing Spin Order

Phil Szuromi

Noncontact atomic force microscopy (ncAFM) can be used to image the spin structure of surfaces when the scanning probe (tip) is fabricated from a ferromagnetic material, such as iron. One target for imaging is the superexchange interaction between oxygen atoms on the surface of the antiferromagnetic nickel oxide, a Mott insulator. For the (001) surface of this oxide, the exchange interaction creates a (2 × 1) spin pattern. This spin structure could be imaged at cryogenic temperatures via magnetic exchange force microscopy with iron tips when a 5-tesla magnetic field was applied to stabilize the tip's magnetization. Pielmeier and Giessibl have imaged the spin structure of this surface with ncAFM without applying magnetic fields at 4.4 kelvin. They used an iron tip in a frequency modulation mode; the tip oscillated at constant amplitudes that were below 0.5 angstrom. They found that with samariumcobalt alloy tips, the spin of the imaging atom of the tip was much more stable, and the spin signal was enhanced by a factor of 3 to 10. With this tip, a height contrast of 0.05 angstrom was seen for different oxygen atoms in the lattice that was the result of indirect superexchange interactions between the tip and subsurface nickel atoms.

Phys. Rev. Lett.110, 266101 (2013).

Psychology

Unconsciously Motivated

Gilbert Chin

Recent energetic discussions in social psychology have focused on methodological aspects of research into unconscious influences on overt behaviors—for example, when participants walk like the elderly after having read passages containing words associated with old age. One consequence of these discussions would be new research programs that advance our understanding of how behavior interacts with cognitive and neural events when participants are unaware. Hepler and Albarracin have entered this arena with a study of how stimuli below the limit of perception can inhibit a behavior (not pressing a button). Without an externally observable outcome, they relied on the amplitude of an event-related brain potential known as the P3 component, which has been shown to index inhibitory control. As their triggers for unconscious processing, they exposed participants to subliminally presented inaction (calm) and action (move) words and ascertained that participants were unaware of these primers. They found that the former set of words increased inhibitory neural activity whereas the latter set decreased it, relative to a control set of neutral words.

Cognition128, 271 (2013).

Ecology

Just Like Home

Sacha Vignieri

CREDIT: STEVE MASLOWSKI

Dispersing juveniles face the daunting task of settling in habitat that will allow them to both survive and reproduce. Our understanding of this habitat selection process is largely based on ideal free distribution (IDF) theory, which proposes that individuals assort within habitats based on the available resources, thus high-quality habitats will attract and support more animals. As with most theories, reality often contradicts IDF patterns, suggesting that other processes are at work. In particular, often individuals colonize a poor habitat even when a good habitat is available. Piper et al. followed individual long-lived common loons from fledging to adulthood over 20 years. They found that dispersers preferentially colonized lakes similar to their natal lakes, particularly in terms of size and alkalinity, even when habitats of higher quality were readily available. This preference was pronounced for the first habitat selected postdispersal and waned later, when animals seemed to select for habitats that facilitated the greatest reproductive success. This temporal change suggests that familiar conditions may be beneficial to young animals as they learn survival skills and that habitat of the highest quality is only important when reproducing becomes a greater challenge than surviving.